I'm booked on AA50 from DFW-LHR waiting on a C seat in sold out Bus. Class. I need to switch flights same day in order to improve my C upgrade chances, by taking either AA78 or AA80. I want to avoid paying the $300 change fee. Can I simply call into EXP desk within 24 hrs of departure of those later flights, and request a same day change and get wait listed for a C seat, without having to pay a $300 change fee?